This invention relates to printing systems. In particular, the invention relates to a printing system which is adapted to produce printed documents with high speed, whereby the printing data are provided from a variable print data source and the printing system is a kind of production line.
In today's high speed printing environment the assurance of document verification and process control is increasing. However, according to a further demand, the printing speed should not be significantly reduced by any inspection techniques. The speed of a single high-speed printer normally exceeds 50 DIN A 4 pages per minute. Its speed may even be some hundreds up to a thousand DIN A 4 pages (images) per minute and—by further development of high speed variable data printers—may even increase to still higher printing speeds.
To further increase the printing speed of variable data printing lines, it has been proposed to perform printing of documents over two or more printers. In particular, this may apply to printers printing on fanfold recording carriers, whereby the carrier already printed by a first printer is subsequently fed to a second printer for a second printing process. The first printer may print onto a first side (front side) of the printing carrier while the second printer may print onto the reverse side of the carrier. Alternatively, the first and second printers may print on the same side of the carrier, but the printers may be loaded with different inks. In particular, electrographic printers such as electrostatographic or magnetographic printers may utilize different toners such as standard optical black toner, colored toner or magnetic ink characters readable toner, which is also known in the art as MICR toner.
A further electrophotographic printer especially adapted for printing on a fanfold recording carrier is described in U.S. Pat. No. 5,778,297.
In many presently available printing systems document verification is not performed at all. However, there have been proposed printing systems with integrated qualification or inspection systems for the printed images. Such a system is described in U.S. Pat. No. 5,235,652. This system comprises a set of measurement modules which have sensors for forming various inspection functions such as image location and spacing measurements; image print contrast/intensity measurement; image skew angle measurements; image stroke with measurement; image edge variation measurement; image void measurement; image size and dimension measurement; image extraneous ink measurement and image curvature measurement.
If such a document verification system is to be introduced into a production printing line, various technical problems occur.